The present invention relates to transition circuits, and more particularly to a variable voltage transition circuit with improved transition characteristics.
An ideal variable voltage transition circuit provides a perfectly linear edge between transition levels with hard clamp at the desired level amplitudes, as shown in FIG. 1. A constant current charging a capacitor yields a voltage which is linear with respect to time. A classical approach to a variable voltage transition circuit is shown schematically in FIG. 2. A charging current source I1 and a discharging current source I2 are alternately switched into a capacitive load C. The slope of the voltage transition is controlled by the magnitude of the current as well as the amount of load capacitance, i.e., dv/dt =I/C. The circuit has several states of operation which are described below with respect to FIG. 3.
Prior to time t1 transistors Q2 and Q3 are turned on along with diode D5, with the current through D5 equal to the current I2 through Q3 to hold the potential of the capacitor C constant, i.e., current I2 flows from voltage rail Vc through transistor Q5 of the clamp circuit, Schottky diode D5 and transistor Q3 to voltage rail Vee, and current I1 flows through transistor Q2 to ground. At time t1 transistors Q1 and Q4 turn on in response to a transition in the differential input signal Vin, and transistors Q2 and Q3 are turned off. However diode D5 continues to conduct until its forward voltage is decreased below its threshold value. Capacitor C initially is charged by current I1+I2 through Q1 and D5, yielding an increased slope relative to a desired slope. In fact the initial slope at time t1 is twice the desired slope and decreases as the forward bias on D5 decreases. At time t2 diode D5 is turned off, and the current I1 in transistor Q1 is equal to the current into the capacitor C so that C charges linearly at the desired slope.
At time t3 the voltage on the capacitor C crosses the threshold required to turn on diode D6. As D6 begins to conduct, current is diverted from the capacitor C and the slope decreases from the desired slope. This decrease becomes more prominent as D6 is forward biased further. The distortion in this time interval t3-t4 lasts longer than that of the time interval t1-t2 due to current being diverted from capacitor C as opposed to capacitor C receiving additional current. Finally at time t4 the amplitude clamp level is reached and the current in transistor Q1 is equal to the current in diode D6, i.e., current I1 flows through transistor Q1 and Schottky diode D6 into transistor Q6 of the opposite clamp circuit while current I2 flows through transistor Q4. The potential across capacitor C is held constant at the new amplitude level.
At time t5, analogous to time t1, transistors Q2 and Q3 turn on and Q1 and Q4 turn off in response to the opposite transition of Vin. However diode D6 continues to conduct until its forward voltage is decreased to below its threshold value. Capacitor C is discharged by Q3 and D6, yielding an increased slope relative to the desired slope The initial slope at t5 is twice the desired value and decreases as the forward bias on D6 decreases. During time t6-t7 when diode D6 is turned off and the current I2 through transistor Q3 is equal to the current from the capacitor C, the capacitor discharges linearly.
At time t7, analogous to time t3, the voltage on the capacitor C is crossing the threshold required to turn on diode D5. As D5 begins to conduct, current is diverted from the capacitor C and the slope of the voltage transition decreases from the desired value. This decrease in slope becomes more prominent as D5 is further forward biased. Finally at time t8 the opposite amplitude level is achieved as defined by the appropriate clamp circuit, and the current in diode D5 is equal to the current I2. The potential across C is held constant at the transition level defined by the clamp circuit. The length of the distorted time intervals, t1-t4 and t5-t8, is directly proportional to the overall transition time.
What is desired is an improved variable voltage transition circuit that virtually eliminates the distortion problems inherent in the prior art approach, as well as providing improved drift characteristics and thermal tail reduction.